17766
J. Phys. Chem. 1996, 100, 17766-17771
Systematic Study of the Photoluminescent and Electroluminescent Properties of Pentacoordinate Carboxylate and Chloro Bis(8-hydroxyquinaldine) Complexes of Gallium(III) Linda S. Sapochak,†,‡ Paul E. Burrows,§ Dimitri Garbuzov,§ Douglas M. Ho,† Stephen R. Forrest,*,§ and Mark E. Thompson*,| Department of Chemistry, Princeton UniVersity, Princeton, New Jersey 08544, AdVanced Technology Center for Photonic and Optoelectronic Materials, Department of Electrical Engineering, Princeton UniVersity, Princeton, New Jersey 08544, and Department of Chemistry, UniVersity of Southern California, Los Angeles, California 90089-0744 ReceiVed: June 17, 1996X
A detailed analysis of the structural, optical absorption, photoluminescent, and electroluminescent properties of a series of pentacoordinate bis(8-hydroxyquinaldine)gallium(III) complexes, i.e. q′2GaX with X ) acetate, dimethylpropionate, benzoate, and chloro, is presented. These materials are compared with tris(8hydroxyquinaldine)Ga and the tris(8-hydroxyquinolate) complexes of aluminum (Alq3) and gallium. Structural studies show that the pentacoordinate complexes have significantly less steric congestion than their hexacoordinate analogues, leading to comparatively smaller Franck-Condon shifts in their absorption and emission spectra. Relatively short π-π stacking distances are observed between the quinolate or quinaldine ligands of adjacent complexes (i.e., 3.3-3.5 Å), which facilitates electron transport in the materials. All of the pentacoordinate complexes had high electroluminescent efficiencies at low drive currents. The benzoate derivative exhibits light output saturation at high drive currents when used in electroluminescent devices, while the other derivatives had characteristics comparable to that of Alq3 at all drive currents.
Introduction Considerable research is currently focused on the development of new light-emitting device technologies for flat panel displays. The primary motivation is to replace bulky and energyconsuming cathode ray tubes (CRTs) with energy-efficient flat panels. While liquid crystal displays (LCDs) are a reasonable substitute for CRTs in some applications, it is advantageous for ease of viewing in bright background environments to have an emissive display, rather than the reflective or transmissive panels of LCDs. One emissive technology that shows promise involves organic light-emitting diodes (OLEDs). The interest in organic molecular materials for use in (OLEDs) began with the report of efficient green electroluminescence (EL) from aluminum tris(8-hydroxyquinoline) (Alq3).1,2 Since those reports, other metal chelate systems3,4 and polymers5 have been found which efficiently produce EL throughout the visible spectrum. Devices prepared with molecular and polymeric materials can be fabricated on virtually any substrate by simple vapor deposition or wet processes, making them economically attractive as well. In this paper we present a systematic study of the optical absorption, photoluminescent (PL), and electroluminescent (EL) properties of a series of pentacoordinate bis(8-hydroxyquinaldine)gallium(III) complexes, which have potential for use as efficient organic light emitters for flat panel displays (FPDs). Results are compared with the hexacoordinate tris(8-hydroxyquinoline) complexes of Al and Ga (Alq3 and Gaq3) and tris(8-hydroxyquinaldine)gallium(III) (Gaq′3). †
Department of Chemistry, Princeton University. Current address: Alpha Analytical, Inc., 2505 Chandler Ave., Suite 1, Las Begas, NV 89120. § Department of Electrical Engineering, Princeton University. | University of Southern California. * To whom correspondence should be addressed. S.R.F.: e-mail,
[email protected]; voice, (609) 258-4532; FAX, (609) 258-1954. M.E.T.: e-mail,
[email protected]; voice, (609) 740-6402; FAX, (609) 740-0930. X Abstract published in AdVance ACS Abstracts, October 1, 1996. ‡
S0022-3654(96)01770-4 CCC: $12.00
Alq3 is the most thoroughly studied molecular emitter material for OLEDs. This hexacoordinate chelate complex exhibits a very high PL quantum efficiency in the solid state (φ ) 0.32).6 The excited state for Alq3 is a localized Frenkel exciton,7 eliminating the intermolecular interactions that can lead to selfquenching. Although a material must be fluorescent in the solid state in order to exhibit electroluminescence, the efficiency of the fluorescence is only one criterion for determining the usefulness of a material as an emitter in an OLED. The criteria for efficient EL include the material volatility, film forming properties, charge transport properties, energy band offsets relative to the carrier transporting electrodes and organic layers, and environmental stability. These parameters are strongly coupled to molecular structure and the bulk molecular packing characteristics. An example of how these parameters can affect electroluminescence is the comparison of Alq3 and Gaq3 OLEDs.8 Even though Alq3 has a 3-4 times larger PL efficiency than Gaq3 in thin film form, their EL efficiencies are comparable. Furthermore, the Gaq3 OLEDs exhibit a lower turn-on voltage than the corresponding Alq3 devices, and thus, a 50% higher power efficiency has been achieved.8 It is important to gain a better understanding of how to optimize these parameters in order to design superior emitter materials. Experimental Section Synthesis and Preparation of Materials. All reagents were obtained from Aldrich Chemical Co. The Ga(NO3)3‚xH2O was high purity grade (99.999%). 8-Hydroxyquinaldine was recrystallized from ethanol/water before use. The syntheses for all of the compounds reported herein are shown schematically in Figure 1. Alq3, Gaq3,9 and phenolatobis(8-hydroxyquinaldine)aluminum(III),10 as well as two of the pentacoordinate q′2GaX complexes (X ) OAc, Cl),11,12 were prepared using published procedures. Spectroscopic and analytical data for these materials matched those reported previously. All of the © 1996 American Chemical Society
Pentacoordinate Gallium(III) Complexes
J. Phys. Chem., Vol. 100, No. 45, 1996 17767
TABLE 1: Crystallographic Coordinates and Thermal Parameters for All Nonhydrogen Atoms of Gaq′3 Ga N(1) C(2) C(3) C(4) C(4a) C(5) C(6) C(7) C(8) O(8) C(8a) C(9) N(1′) C(2′) C(3′) C(4′) C(4a′) C(5′) C(6′) C(7′) C(8′) O(8′) C(8a′) C(9′) N(1′′) C(2′′) C(3′′) C(4′′) C(4a′′) C(5′′) C(6′′) C(7′′) C(8′′) O(8′′) C(8a′′) C(9′′)
x
y
z
U(eq)a
3012(1) 2746(2) 2270(2) 2248(2) 2696(2) 3201(2) 3688(2) 4150(2) 4156(2) 3691(2) 3676(1) 3203(2) 1756(2) 3580(2) 3627(2) 4111(2) 4550(2) 4526(2) 4960(2) 4891(2) 4395(2) 3964(2) 3495(1) 4020(2) 3176(3) 2115(2) 1994(2) 1367(2) 866(2) 971(2) 497(2) 650(3) 1280(2) 1762(2) 2359(1) 1611(2) 2519(3)
5481(1) 5960(2) 5774(3) 6196(3) 6807(3) 7024(3) 7655(3) 7828(3) 7386(3) 6765(2) 6353(2) 6576(3) 5115(3) 5013(2) 5239(2) 4863(3) 4284(3) 4041(3) 3449(3) 3264(3) 3651(3) 4238(2) 4636(2) 4427(2) 5916(3) 4561(2) 3755(3) 3387(3) 3842(3) 4696(3) 5209(3) 6020(3) 6365(3) 5880(3) 6176(2) 5019(3) 3219(3)
2183(1) 712(2) -269(3) -1066(3) -878(3) 131(3) 419(3) 1426(3) 2196(3) 1970(3) 2683(2) 914(3) -510(3) 3717(2) 4576(3) 5520(3) 5599(3) 4713(3) 4705(3) 3805(3) 2883(3) 2853(3) 1998(2) 3786(3) 4521(3) 1688(2) 1446(3) 1137(3) 1067(3) 1344(3) 1347(4) 1614(4) 1892(3) 1905(3) 2164(2) 1641(3) 1471(5)
34(1) 35(2) 43(2) 49(2) 49(3) 40(2) 50(3) 49(3) 41(2) 34(2) 40(1) 34(2) 60(3) 31(2) 36(2) 41(2) 44(2) 37(2) 44(2) 47(3) 41(2) 34(2) 36(1) 29(2) 53(3) 37(2) 45(2) 54(3) 56(3) 44(2) 60(3) 64(3) 53(3) 39(2) 40(2) 36(2) 70(4)
materials were purified by vacuum sublimation at 225 °C at 10-4 Torr, prior to OLED fabrication. Elemental analyses for all of the materials were obtained from Robertson Microlit Laboratories, Inc., Madison, NJ. Bis(8-hydroxyquinaldine)gallium 2,2-dimethylpropionate (q′2GaDMP) was prepared by the following procedure. A solution of 2.0 g (7.82 mmol) of Ga(NO3)3‚xH2O in 200 mL of water was stirred rapidly with warming, while a solution of 2.452 g (15.4 mmol) of 8-hydroxyquinaldine and 7.99 g (78.2 mmol) of 2,2-dimethylpropionic acid in 600 mL of water was added over 1 h. A precipitate formed immediately, and stirring was continued for an additional 1 h after all of the reactants had been added. The precipitate was filtered, washed with hot water, air-dried, and recrystallized from methanol to yield 1.03 g (35%) of large green-yellow crystals, mp ) 264-265 °C. 1H NMR [250 MHz, CDCl3 data reported as assignment ) δ in ppm (multiplicity) (coupling constant in Hz)]: H3 ) 7.42 (d) (8.55); H4 ) 8.24 (d); H5 ) 7.14 (d) (8.24); H6 ) 7.43 (t) (7.93); H7 ) 7.10 (d); CH3-q′ ) 3.03 (s); (CH3)3CCO2 ) 1.02 (s). FT-IR (KBR): 1652, 1507, 1465, 1457, 1431, 1394, 1344, 1272, 1116, 833, 756, 528. Anal. Calcd for C25H25GaN2O4: C, 61.64; H, 5.17; N, 5.75. Found: C, 61.44; H, 5.13; N, 5.74. Bis(8-hydroxyquinaldine)gallium benzoate (q′2GaBen) was prepared by the following procedure. A solution of 1.0 g (3.9 mmol) of Ga(NO3)3‚xH2O in 50 mL of water was stirred rapidly while a solution of 1.226 g (7.7 mmol) of 8-hydroxyquinaldine and 6.185 g (38.6 mmol) of potassium benzoate in 280 mL of ethanol/water (50:50) was added over 30 min. Precipitation of the product occurred immediately, and the reaction mixture was stirred an additional 30 min after all the reactants had been
Figure 1. Synthetic scheme for the q′2GaX complexes investigated in this study.
added. The precipitate was filtered, washed with ethanol, airdried, and recrystallized from chlorobenzene to yield 1.982 g (50%) of small green-yellow crystals, mp ) 340 °C (dec). 1H NMR [250 MHz, CDCl3 data reported as assignment ) δ in ppm (multiplicity) (coupling constant in Hz)]: H3 ) 7.39 (d) (8.55); H4 ) 8.24 (d); H5 ) 7.16 (d) (8.24); H6 ) 7.46 (t) (7.93); H7 ) 7.15 (d); CH3-q′ ) 3.04 (s); H8-ArCO2 ) 7.92 (d) (7.02); H9-ArCO2 ) 7.27 (t) (6.41); H10-ArCO2 ) 7.357.50 (m). FT-IR (KBR): 1634, 1610,1509, 1467, 1453, 1433, 1389, 1340, 1305, 1274, 1112, 839, 755, 729, 693, 649, 528. Anal. Calcd for C27H21GaN2O4: C, 63.94; H, 4.17; N, 5.52. Found: C, 63.81; H, 4.11; N, 5.55. X-ray Crystallographic Characterization of Gaq′3. The X-ray diffraction analysis of Gaq′3 was performed on a Siemens P4 four-circle diffractometer employing graphite-monochromated Mo KR radiation (λ ) 0.710 73 Å). Unit cell constants were determined by a least-squares fit of the 2θ values for 25 centered reflections having 25° < 2θ < 39°. Systematic absences were consistent with the monoclinic space group C2/ c. A total of 4683 reflections were measured (ω scan mode, 4° < 2θ < 50°) and corrected for Lorentz-polarization and absorption effects. Of these, 4394 were unique (Rint ) 1.32%) and 3116 reflections having F > 3σ(F) were considered observed. The structure of Gaq′3 was solved by direct methods and refined using the SHELXTL Plus package of programs. All of the nonhydrogen atoms were refined with anisotropic displacement coefficients; hydrogen atoms were included with a riding model [C-H ) 0.96 Å, U(H) ) 1.2U(C)]. The methyl hydrogen atoms were located in a difference-Fourier map and idealized, and each CH3 group was refined as a rigid group. The refinement converged to R ) 4.60%, wR ) 4.91%, and S ) 1.08, with 343 variables and 9.1 reflections per refined parameter. The atom positions for all nonhydrogen atoms are given in Table 1, and a thermal ellipsoid plot for Gaq′3 is shown in Figure 2. The unit cell for Gaq′3 (C30H24GaN3O3) was refined
17768 J. Phys. Chem., Vol. 100, No. 45, 1996
Sapochak et al. recorded with an EG&G optical multichannel analyzer on a 0.25 focal length spectrograph. Results and Discussion
Figure 2. Thermal ellipsoid figure of Gaq′3.
to a ) 23.712(3) Å, b ) 16.057(2) Å, c ) 15.722(2) Å, β ) 124.049(8)°, volume ) 4959.9(10) Å3, Z ) 8, and density (calcd) ) 1.458 g/cm3. Optical Characterization. Absorption spectra were recorded with a Hewlett-Packard spectrophotometer. Photoluminescent and excitation spectra were obtained with a Perkin-Elmer LS100 Fluorimeter for both chloroform solution (filtered and degassed) and thin film samples on quartz (thin films were prepared as described previously).7,8 The emission spectra were recorded using an excitation wavelength of 365 nm, and the excitation spectra were recorded by detecting emission at 500 nm. Alq3 was used as the reference for quantum yield (φPL) calculations based on the reported φPL(Alq3) in chloroform (0.04).13 The thin film φPL for q′2GaCl was measured as described previously.6 Devices were grown on glass slides precoated with ITO (sheet resistance of 15 Ω/square). Substrates were ultrasonically cleaned in detergent solution for about 1 min, followed by thorough rinsing in deionized water. They were then boiled in 1,1,1-trichloroethane, rinsed in acetone followed by methanol, and dried in pure nitrogen gas between each step. Devices were formed by sequential, high vacuum ( Alq3 > q′2GaCl > q′2GaOAc > Gaq3 (Figure 5a). At higher drive currents (>100 µA), the order is significantly different, where the EL output at a given current level decreases in the order Alq3 > q′2GaDMP > q′2GaCl > Gaq3 > q′2GaOAc > q′2GaBen (Figure 5b). The q′2GaBen and q′2GaDMP complexes are significantly brighter than Alq3 and Gaq3 at low drive currents; however, while the q′2GaDMP complex consistently exhibits an increase in EL output with increasing drive current, the q′2GaBen complex exhibits saturation of the EL output above 200 µA. This saturation is completely reversible, showing no decay or hysteresis on cycling between 0 and 300 µA. The saturation behavior for the q′2GaBen complex and the lower efficiencies for the other q′2GaX complexes may be an intrinsic property of the material (low electron mobility or high heterojunction energy barriers) or may be due to impurities. While the materials were purified by sublimation prior to device fabrication, we cannot rule out that the differences are extrinsic in nature, i.e. they are due to impurities. However, if impurities
Figure 5. Light output versus voltage plots for Alq3, Gaq3, and q′2GaX OLEDs.
are responsible for the saturation behavior, It is interesting to note that they do not lead to significant fluorescence quenching. Further purification procedures are currently underway, and reanalysis of these complexes is warranted to better understand their device properties. We are also investigating the crystal structures of the q′2GaX complexes to see if there are any obvious relationships between the intermolecular interactions and device properties. Conclusions The PL and EL spectra for the q′2GaX complexes (X ) Cl, OAc, DMP, Ben) are very similar and are blue-shifted substantially from those of Alq3 and Gaq3. The solution PL quantum efficiencies determined for the q′2GaX complexes are greater than those reported for Alq3 and Gaq3. As observed for Mq3based devices; however, an increase in PL efficiency does not lead directly to an increase in EL efficiency. While all of the q′2GaX-based OLEDs performed well relative to Alq3 and Gaq3 at low drive currents, at higher drive currents, the Mq3 devices showed higher EL quantum efficiencies. The q′2GaBen OLED has the best EL quantum efficiency at low drive currents but shows saturation behavior at higher currents. Acknowledgment. The authors thank the Universal Display Corp., ARPA (Contract No. F33615-94-1-1414), and the National Science Foundation (Grant No. DMR94-00362) for their financial support of this work. References and Notes (1) Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 1987, 51, 913. (2) Tang, C. W.; VanSlyke, S. A. J. Appl. Phys. 1989, 65, 3610.
Pentacoordinate Gallium(III) Complexes (3) Hamada, Y.; Sano, T.; Fujita, M.; Fujii, T.; Nishio, Y.; Shibata, K. Jpn. J. Appl. Phys. 1993, 32, L514. (4) Adachi, C.; Tsutsui, T.; Saito, S. Appl. Phy. Lett. 1990, 56, 799. (5) Holmes, A. B.; Bradley, D. D. C.; Brown, A. R.; Burn, P. L.; Burroughes, J. H.; Friend, R. H.; Greenham, N. C.; Gymer, R. W.; Halliday, D. A.; Jackson, R. W.; Kraft, A.; Martens, J. H. F.; Pichler, K.; Samuel, I. D. W. Synth. Met. 1993, 55-57, 4031. Buchwald, E.; Meier, M.; Karg, S.; Po¨sch, P.; Schmidt, H. W.; Strohiergl, P.; Reiss, W.; Schwoerer, M. AdV. Mat. 1995, 7, 839. Kido, J.; Komada, M.; Harada, G.; Nagai, K. Polym. AdV. Technol. 1995, 6, 703. Yang, Z.; Hu, B.; Karasz, R. E. Macromolecules 1995, 28, 6151. Greenham, N. C.; Moratti, S. C.; Bradley, D. D. C.; Friend, R. H.; Holmes, A. B. Nature 1993, 365, 628. Parker, I. D. J. Appl. Phys. 1994, 75, 1656 and references therein. (6) Garbuzov, D. Z.; Bulovic, V.; Burrows, P. E.; Forrest, S. R. Chem. Phys. Lett. 1996, 249, 433. (7) Burrows, P. E.; Shen, Z.; Bulovic, V.; McCarty, D. M.; Forrest, S. R.; Cronin, J.; Thompson, M. E. J. Appl. Phys., in press. (8) Burrows, P. E.; Sapochak, L. S.; McCarty, D. M.; Forrest, S. R.; Thompson, M. E. Appl. Phys. Lett. 1994, 64, 2718. (9) Schmidbaur, H.; Lettenbauer, J.; Wilkinson, D. L.; Muller, G.; Kumberger, O. Z. Naturforsch. 1991, 46b, 901. (10) Van Slyke, S. A., et al. U.S. Patent 5,150,006, 1991.
J. Phys. Chem., Vol. 100, No. 45, 1996 17771 (11) Schmidbauer, H.; Lettenbauer, J.; Kumberger, O.; Lachmann, J.; Muller, G. Z. Naturforsch. 1991, 46b, 1065. (12) Shiro, M.; Fernando, Q. Anal. Chem. 1971, 43, 1222. (13) Ohnesorge, W. E.; Rogers, L. B. Spectrochem. Acta 1959, 27. (14) Ho, D. M.; Sapochak, L. S. Private communication. (15) Popovych, O.; Rogers, L. B.; Spectrochim. Acta 1960, 49. (16) Popovych, O.; Rogers, L. B.; Spectrochim. Acta 1965, 1229. (17) Suzuki, H. Electronic Absorption Spectra and Geometry of Organic Molecules; Academic Press: New York, 1967; pp 82-87. (18) Skoog, P. A. Principles of Instrumental Analysis; Saunders College Pub.: New York, 1985. (19) Turro, N. J. Modern Molecular Photochemistry; Benjamin/Cummings Publishing Co., Inc.: Menlo Park, CA, 1978. (20) Guillet, J. Polymer Photophysics and Photochemistry; Cambridge University Press: Cambridge, 1985. (21) Anderson, W. P.; Edwards W. D.; Zerner, M. C. Inorg. Chem. 1986, 25, 2728. Anderson, W. P.; Cundari, T. R.; Drago, R. S.; Zerner, M. C. Inorg. Chem. 1990, 29, 1. Kotzian, M.; Rosch, N.; Schroeder, H.; Zerner, M. C. J. Am. Chem. Soc. 1989, 111, 7687. ZINDO was run on a Silicon Graphics, Indigo-2 workstation, using Biosym International software.
JP961770T
